1. Field of the Invention
The present invention relates to methods for manufacturing a semiconductor device, methods for manufacturing a display device, semiconductor devices, display devices, and electronic devices.
2. Description of the Related Art
In recent years, flat panel displays such as liquid crystal display devices and electroluminescence (EL) display devices have attracted attention.
Driving methods of the flat panel displays include a passive matrix method and an active matrix method. An active matrix method has advantages over a passive matrix method in low power consumption, high definition, a large-sized substrate, and the like.
Here, a structural example of an active matrix display device is briefly described with reference to FIGS. 14A and 14B. FIG. 14A illustrates an example of a structure in which a driver circuit is provided using a tape automated bonding (TAB) method.
In FIG. 14A, a pixel portion 1401 in which pixels 1402 are arranged in matrix is formed over a substrate 1400 having an insulating surface. The pixels 1402 are arranged in matrix at intersections of scan lines extending from a scan line side input terminal 1403 and signal lines extending from a signal line side input terminal 1404. Each of the pixels in the pixel portion 1401 is provided with a switching element and a pixel electrode layer connected to the switching element. A typical example of the switching element is a thin film transistor (TFT). A gate electrode layer side of a TFT is connected to a scan line, and a source or drain side of the TFT is connected to a signal line. In addition, an integrated circuit (IC) 1451 included in a driver circuit is connected to a flexible printed circuit (FPC) 1450.
In a structure where a driver circuit is provided outside as in FIG. 14A, it is possible to use, as a driver circuit, an IC using a single-crystal silicon; therefore, a problem due to a speed of a driver circuit does not arise. However, when an IC is provided in this manner, the manufacturing cost cannot be reduced sufficiently because of necessity of preparing a display portion and an IC separately, necessity of a step of connecting the display portion and the IC, and the like. Further, there has been a problem in that a display device increases in size (particularly thickness) because of an IC.
Thus, in view of solution to the above-described problems, a method in which a pixel portion and a driver circuit are formed over the same substrate has been employed (for example, see Patent Document 1: Japanese Published Patent Application No. H8-6053). FIG. 14B illustrates an example of a structure in which a pixel portion and a driver circuit portion are formed over the same substrate.
In the case illustrated in FIG. 14B, non-single-crystal silicon such as amorphous silicon, microcrystalline silicon, or polycrystalline silicon is used as a semiconductor layer of a driver circuit 1460, similar to the pixel portion. However, even in a case where microcrystalline silicon or polycrystalline silicon as well as amorphous silicon is used, there is a problem in that characteristics thereof are incomparable to characteristics of single-crystal silicon. In particular, in a semiconductor layer which is used for a conventional display device where a driver circuit is integrated, it has been difficult to manufacture a TFT having characteristics (mobility, a subthreshold swing (S value), and the like), which are necessary and sufficient. This has caused a big problem in manufacturing a semiconductor device where high speed operation is required, that is, a driver circuit.